The present invention generally relates to the field of electric power transformer systems, and particularly relates to such systems that use an internal fluid, such as oil, for dielectric and/or heat transfer purposes.
Oil is used extensively as an internal fluid in modern electric power apparatus, for example in power transformers of most transmission systems. Oil generally serves two necessary functions; to insulate voltage between metallic conductors at close clearances, and to aid in the removal of heat generated by losses in the conductors. Unfortunately, oil can degrade and loose it's ability to adequately insulate voltage and thereby enhance the risk of a dielectric failure; this in-turn can lead to a forced outage and/or catastrophic failure. The direct dollar cost of a single failure may be very expensive (possibly exceeding ten million dollars) and may further cause adverse implications for safety and the environment.
It is known to employ regular inspection of transformer oil condition, often on an annual basis and especially in oil insulated tap changer compartments. Such inspections typically involve careful manual extraction of an oil sample and subsequent testing in a dielectric strength tester. The dielectric strength test is typically made in accordance with a recognized standard procedure and apparatus, typically via standards such as the American Society of Testing Materials (ASTM) D-877 or D-1816 or certain international standards such as provided by IEC/VDE authorities. Such tests apply an AC voltage stress to an electrode pair immersed in the subject oil, the stress is continuously increased until breakdown occurs. The voltage at breakdown is the measure of insulating ability. These standard tests employ defined electrodes, defined applied AC high voltage at fixed rates of rise until breakdown, and a statistical method for interpreting the results of a few repeated tests.
Unfortunately such conventional test must be performed on a sample because they are destructive to the oil. The sample measured is discarded after the test. This means that the test cannot be used on oil directly inside an operating power apparatus. The oil must be extracted so as not to compromise the integrity of the operating unit. The costs and limitations imposed by manual sampling and measurement require that dielectric strength tests are performed typically on an annual or semiannual basis. This may permit more rapid degradation to be undetected in some cases, resulting in occasional failures.
In an attempt to achieve in-service measurement, two techniques have been employed. One uses an active detection and fast switch device to detect the onset of a discharge and to rapidly interrupt or divert the supply source. This is limited in effectiveness because of the necessity for reliable and very fast interruption of high-voltages, a difficult task. A failure of any of the interruption mechanism would result in degradation of the oil and possible faulting of the power apparatus being tested.
A second technique, referred to as the Non-Destructive Breakdown method, NDBD, employs a pulsed high-voltage applied to electrodes in an oil test gap. The gap is selected such that there is no breakdown event for the NDBD pulse voltage for good oil condition; however as the oil condition degrades then NDBD applied voltage induces breakdown events. The oil condition is assessed from the NDBD breakdown activity. Three major differences that distinguish the NDBD measurement from the ASTM—type dielectric strength tests are (1) it uses time-lag to breakdown at pulse voltage, instead of a 60 Hz AC voltage raised to BD, (2) it uses a short (submicrosecond) duration constant pulse voltage from a pulser; and (3) it use a negative point-to-plane electrode, instead of ASTM symmetric planar electrodes.
To reduce the effects of statistical variations inherent in the breakdown process, the ASTM test specifies repeated measurements under fixed conditions to more clearly distinguish degradation. Because of changes to the oil due to damage induced by the breakdowns in the test itself however, there is typically a limit of 5 breakdowns maximum on any oil sample. In contrast, the NDBD test allows many more repeated measurements and thus greater accuracy of the measurement.
It is important to recognize that in the NDBD approach the test induces no breakdown events in clean (or good) oil and hence no damage to the oil is possible when the oil is clean. However, when the oil does degrade, detection is achieved by the onset of breakdown activity of the NDBD test events. The occurrence of repeated breakdowns is one signature of a degraded oil condition. Hence it is an essential part of the oil condition assessment that the NDBD test will exhibit progressively greater probability of breakdown in any series of tests as the oil condition significantly degrades. The NDBD test breakdowns must, therefore, not themselves be the cause of significant degradation to the oil, especially for the NDBD test to be acceptable as a measurement performed on oil inside a power apparatus, without oil extraction.
Although conventional NDBD tests on samples yield improvements in reduced oil degradation compared to the ASTM-type tests, the NDBD tests are not suitable for in-service testing, i.e., during operation of the electric power apparatus. The difficultly with performing in-service tests, is that some degradation of the oil may result from the testing itself. Such damage is related to the currents in the oil during breakdown. The characteristic surge impedance of the pulser voltage acts to provide an inherent limit to the current and thereby possible damage to the oil under test. For example, for a 20 kV pulse voltage and a 50 ohm line impedance, a current limit of 400 amperes is achieved for the very short duration of the pulse, e.g., up to 0.3 microseconds. The charge transfer could therefore attain a value of about 120 microcoulombs for this case. The integral of current squared times time is another measure used to quantify the activity of a discharge and for this case amounts to about 5×10−2 amps2-seconds. These values are typically ten times or more smaller than breakdown events from a classical ASTM type test, but still provide conditions in which damage to the oil may result.
There remains a need, therefore, for a system and method for achieving in-service measurement of the dielectric strength of oil in an electric power apparatus.